Fusion bonding (also known as direct bonding) is a process for joining surfaces without intermediate layers. The process is based on the formation of chemical bonds between the surfaces when the surfaces are sufficiently flat, clean, and smooth. Fusion bonding has many applications in the semiconductor manufacturing industry. For example, fusion bonding may be used to package microelectromechanical systems (MEMS) devices, such as accelerometers, pressure sensors, and gyroscopes, or to manufacture semiconductor substrates, such as silicon-on-insulator (SOI) substrates.